This document summarizes research on using nitrogen-containing carbon nanotubes as supports for platinum nanoparticles as an alternative anode catalyst for direct methanol fuel cells. Key points: 1. Nitrogen-containing carbon nanotubes were synthesized using an alumina membrane template and pyrolysis of polyvinylpyrrolidone polymer. 2. Highly dispersed platinum nanoparticles around 3nm in size were uniformly deposited on the nitrogen-containing carbon nanotubes. 3. Electrochemical testing found the platinum catalyst supported on nitrogen-containing carbon nanotubes had over 10 times higher catalytic activity for methanol oxidation compared to a commercial platinum on carbon catalyst.

1. Electrochemistry Communications 7 (2005) 905–912
www.elsevier.com/locate/elecom
Nitrogen containing carbon nanotubes as supports for
Pt – Alternate anodes for fuel cell applications
T. Maiyalagan, B. Viswanathan *, U.V. Varadaraju
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
Received 7 June 2005; accepted 7 July 2005
Available online 8 August 2005
Abstract
Aligned nitrogen containing carbon nanotubes have been synthesized using Anodisc alumina membrane as template. Highly dis-
persed platinum nanoparticles have been supported on the nitrogen containing carbon nanotubes. Nitrogen containing carbon
nanotubes as platinum catalyst supports were characterized by electron microscopic technique and electrochemical analysis. The
EDX patterns show the presence of Pt and the micrograph of TEM shows that the Pt particles are uniformly distributed on the
surface of the nitrogen containing carbon nanotube with an average particle size of 3 nm. Cyclic voltammetry studies revealed a
higher catalytic activity of the nitrogen containing carbon nanotube supported Pt catalysts.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Nitrogen containing carbon nanotubes; Template synthesis; Alumina template; Catalyst support; Methanol oxidation
1. Introduction Pt based catalysts. High noble metal loadings on the
electrode [4,5] and the use of perfluorosulfonic acid
Since the last decade, fuel cells have been receiving an membranes significantly contribute to the cost of the de-
increased attention due to the depletion of fossil fuels vices. An efficient way to decrease the loadings of pre-
and rising environmental pollution. Fuel cells have been cious platinum metal catalysts and higher utilization of
demonstrated as interesting and very promising alterna- Pt particles is by better dispersion of the desired metal
tives to solve the problem of clean electric power gener- on the suitable support [6]. In general, small particle size
ation with high efficiency. Among the different types of and high dispersion of platinum on the support will re-
fuel cells, direct methanol fuel cells (DMFCs) are excel- sult in high electrocatalytic activity. Carbon materials
lent power sources for portable applications owing to its possess suitable properties for the design of electrodes
high energy density, ease of handling liquid fuel, low in electrochemical devices. Carbon is an ideal material
operating temperatures (60–100 °C) and quick start up for supporting nano-sized metallic particles in the elec-
[1,2]. Furthermore, methanol fuel cell seems to be highly trode for fuel cell applications. No other material except
promising for large-scale commercialization in contrast carbon material has the essential properties of electronic
to hydrogen-fed cells, especially in transportation [3]. conductivity, corrosion resistance, surface properties,
The limitation of methanol fuel cell system is due to and the low cost required for the commercialization of
low catalytic activity of the electrodes, especially the an- fuel cells. In general, the conventional supports namely
odes and at present, there is no practical alternative to carbon black is used for the dispersion of Pt particles [7].
The appearance of novel carbon support materials,
*
Corresponding author. Tel.: +91 044 22574200; fax: +91 44
such as graphite nanofibers (GNFs) [8,9], carbon nano-
22574202. tubes (CNTs) [10–17], carbon nanohorns [18], and car-
E-mail address: bvnathan@iitm.ac.in (B. Viswanathan). bon nanocoils [19–22], provides new opportunities of
1388-2481/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.elecom.2005.07.007

2. 906 T. Maiyalagan et al. / Electrochemistry Communications 7 (2005) 905–912
carbon supports for fuel cell applications. Bessel et al. [8] alumina template by wetting method [28]. After
and Steigerwalt et al. [9] used GNFs as supports for Pt complete solvent evaporation, the membrane was placed
and Pt–Ru alloy electrocatalysts and observed better in a quartz tube (30 cm length, 3.0 cm diameter), kept in
activity for methanol oxidation. The high electronic a tubular furnace and carbonized at 1173 K under Ar
conductivity of GNFs and the specific crystallographic gas flow. After 3 h of carbonization, the quartz tube
orientation of the metal particles resulting from well- was cooled to room temperature. The resulting template
ordered GNF support were believed to be the important with carbon–nitrogen composite was immersed in 48%
factors for the observed enhanced electrocatalytic activ- HF at room temperature for 24 h to remove the alumina
ity. The morphology and the nature of the functional template and the nitrogen containing CNTs were
groups of the support influence the activity of fuel cell obtained as an insoluble fraction. The nanotubes were
electrocatalyts [23–26]. Carbon with sulphur or nitrogen then washed with distilled water to remove the residual
based functionality [25], can influence the activity of the HF and dried at 393 K.
catalyst.
The present report focuses on the efforts undertaken 2.3. Loading of Pt catalyst inside nanotube
to develop unconventional supports based platinum
catalysts for methanol oxidation. Nitrogen containing Platinum nanoclusters were loaded inside the N-CNT
carbon nanotubes were used to disperse the platinum as follows; the C/alumina composite obtained (before
particles effectively without sintering and to increase the dissolution of template membrane) was immersed
the catalytic activity for methanol oxidation. The tubu- in 73 mM H2PtCl6 (aq) for 12 h. After immersion, the
lar morphology and the nitrogen functionality of the membrane was dried in air and the ions were reduced
support have influence on the dispersion as well as to the corresponding metal(s) by a 3 h exposure to flow-
the stability of the electrode. In this communication ing H2 gas at 823 K. The underlying alumina was then
the preparation of highly dispersed platinum supported dissolved by immersing the composite in 48% HF for
on nitrogen containing carbon nanotubes, the evalua- 24 h. This procedure resulted in the formation of Pt
tion of the activity for the methanol oxidation of these nanocluster loaded N-CNT and the complete removal
electrodes and comparison with the activity of conven- of fluorine and aluminum was confirmed by EDX
tional electrodes are reported. analysis.
2.4. Preparation of working electrode
2. Experimental
Glassy carbon (GC) (Bas electrode, 0.07 cm2) was
2.1. Materials polished to a mirror finish with 0.05 m alumina suspen-
sions before each experiment and served as an underly-
All the chemicals used were of analytical grade. Poly- ing substrate of the working electrode. In order to
vinyl pyrrolidone (Sisco Research Laboratories, India), prepare the composite electrode, the nanotubes were
dichloromethane and concentrated HF (both from dispersed ultrasonically in water at a concentration of
Merck) were used. Hexachloroplatinic acid was ob- 1 mg mlÀ1 and 20 ll aliquot was transferred on to a
tained from Aldrich. 20 wt% Pt/Vulcan carbons were polished glassy carbon substrate. After the evaporation
procured from E-TEK. Methanol and sulphuric acid of water, the resulting thin catalyst film was covered
were obtained from Fischer chemicals. The alumina with 5 wt% Nafion solution. Then the electrode was
template membranes (Anodisc 47) with 200 nm diameter dried at 353 K and used as the working electrode.
pores were obtained from Whatman Corp. Nafion
5 wt% solution was obtained from Dupont and was used 2.5. Characterization methods
as received.
The chemical composition of the nanotubes was
2.2. Synthesis of nitrogen containing carbon nanotubes determined by elemental analysis using Hereaus CHN
analyzer after the removal of alumina template. The
Pyrolysis of nitrogen containing polymers is a facile scanning electron micrographs were obtained using
method for the preparation of carbon nanotube materi- JEOL JSM-840 model, working at 15 keV. The nano-
als containing nitrogen substitution in the carbon frame- tubes were sonicated in acetone for 20 min and then
work. Nitrogen containing carbon nanotubes were were dropped on the cleaned Si substrates. The AFM
synthesized by impregnating polyvinylpyrrolidone imaging was performed in air using the Nanoscope IIIA
(PVP) inside the alumina membrane template and subse- atomic force microscope (Digital Instruments, St. Bar-
quent carbonization of the polymer [27]. Polyvinylpyr- bara, CA) operated in contact mode. For transmission
rolidone (PVP – 5 g) was dissolved in dichloromethane electron microscopic studies, the nanotubes dispersed
(20 ml) and impregnated directly in the pores of the in ethanol were placed on the copper grid and the

3. T. Maiyalagan et al. / Electrochemistry Communications 7 (2005) 905–912 907
images were obtained using Phillips 420 model, operat- nanotubes is shown in Fig. 1(a). Fig. 1(b) shows lateral
ing at 120 keV. view of the nitrogen containing carbon nanotubes with
low magnification and Fig. 1(c) shows lateral view of
2.6. Electrochemical measurements the nitrogen containing carbon nanotubes with high
magnification. The hollow structure and well alignment
All electrochemical studies were carried out using a of the nitrogen containing carbon nanotubes have been
BAS 100 electrochemical analyzer. A conventional identified by SEM.
three-electrode cell consisting of the GC (0.07 cm2) AFM images of the nitrogen containing carbon nano-
working electrode, Pt plate (5 cm2) as counter electrode tubes deposited on a silicon substrate are shown in
and Ag/AgCl reference electrode were used for the cyclic Fig. 2. The AFM tip was carefully scanned across the
voltammetry (CV) studies. The CV experiments were tube surface in a direction perpendicular to the tube
performed using 1 M H2SO4 solution in the absence axis. From the AFM images, it is inferred that a part
and presence of 1 M CH3OH at a scan rate of of the long nanotube is appearing to be cylindrical in
50 mV sÀ1. All the solutions were prepared by using ul- shape and is found to be terminated by a symmetric
tra pure water (Millipore, 18 MX). The electrolytes were hemispherical cap. Because of the finite size of the
degassed with nitrogen gas before the electrochemical AFM tip, convolution between the blunt AFM tip and
measurements. the tube body will be giving rise to an apparently greater
lateral dimension than the actual diameter of the tube
[29].
3. Results and discussion The TEM images are shown in Fig. 3(a)–(c). The
open end of the tubes observed by TEM shows that
Elemental analysis was conducted to examine the nanotubes are hollow and the outer diameter of
whether nitrogen has really entered the carbon nanotube the nanotube closely match with the pore diameter of
framework. It has been found that the samples prepared template used, with a diameter of 200 nm and a length
contained about 87.2% carbon and 6.6% nitrogen (w/w). of approx. 40–50 lm. It is evident from the micrographs
The SEM images of the nitrogen containing carbon that there is no amorphous material present in the nano-
nanotubes support are shown in Fig. 1(a)–(c). Top view tube. Fig. 3(c) shows the TEM image of Pt nanoparticles
of the vertically aligned nitrogen containing carbon filled carbon nanotubes. TEM pictures reveal that the Pt
Fig. 1. SEM images of the nitrogen containing carbon nanotubes: (a) the top view of the nanotubes; (b) side view of the vertically aligned nanotubes
and (c) high magnification lateral view of the nanotubes.

4. 908 T. Maiyalagan et al. / Electrochemistry Communications 7 (2005) 905–912
Fig. 2. AFM image of the nitrogen containing carbon nanotubes.
Fig. 3. TEM images of the nitrogen containing carbon nanotubes: (a) at lower magnification; (b) at higher magnification image of the individual
nanotube (an arrow indicating the open end of the tube) and (c) Pt filled nitrogen containing carbon nanotubes.

5. T. Maiyalagan et al. / Electrochemistry Communications 7 (2005) 905–912 909
particles have been homogeneously dispersed on the the current increases quickly due to dehydrogenation
nanotubes and particle sizes were found to be around of methanol followed by the oxidation of absorbed
3 nm. The optimal Pt particle size for reactions in the methanol residues and reaches a maximum in the poten-
H2/O2 fuel cell is 3 nm [30]. The importance of the Pt tial range between 0.8 and 1.0 V vs. Ag/AgCl. In the
particle size on the activity for methanol oxidation is cathodic scan, the re-oxidation of methanol is clearly
due to the structure sensitive nature of the reaction observed due to the reduction of oxide of platinum.
and the fact that particles with different sizes will Electrocatalytic activity of methanol oxidation has been
have different dominant crystal planes and hence the dif- found to be strongly influenced by the metal dispersion.
ferent intercrystallite distances, which might influence Pure Pt electrode shows an activity of 0.167 mA cmÀ2.
methanol adsorption. The commercial Pt/C has a very The Pt/N-CNT shows a higher activity of 13.3 mA cmÀ2
high specific surface area but contributed mostly by where as conventional 20% Pt/Vulcan (E-TEK)
micropores less than 1 nm and are therefore more diffi- electrode shows less activity of 1.3 mA cmÀ2 compared
cult to be fully accessible. It has been reported that the to nitrogen containing carbon nanotube supported
mean value of particle size for 20% Pt/Vulcan electrode. The nitrogen containing carbon nanotube
(E-TEK) catalyst was 2.6 nm [31]. The EDX pattern of supported electrodes shows a ten fold increase in the
the as synthesized catalyst shows the presence of Pt catalytic activity compared to the E-TEK electrode.
particles in the carbon nanotubes and also the complete The Pt/N-CNT electrode showed higher electrocatalytic
removal of fluorine and aluminum has also been activity for methanol oxidation than commercial
confirmed in Fig. 4. It has been reported that the Pt/Vulcan (E-TEK) electrode. The anodic current
electronic and physical structures of a Pt particle density of Pt/N-CNT electrode is found to be higher
deposited on carbon differ from those of the bulk Pt. than that of Pt/Vulcan (E-TEK) electrode, which indi-
The electronic change in Pt/C is considered as a result cates that the catalyst prepared with nitrogen containing
of functional groups of the carbon support that might carbon nanotubes as the support has excellent catalytic
influence the electronic structure of Pt particulate activity on methanol electrooxidation.
[32–36]. The nitrogen functional group on the carbon The onset potential and the forward-scan peak current
nanotubes surface intensifies the electron withdrawing density for the different electrodes are given in Table 1.
effect against Pt and the decreased electron density of The onset potential of methanol oxidation at nitrogen
platinum facilitate oxidation of methanol. containing carbon nanotube supported catalysts occurs
Fig. 5(a)–(c) shows the cyclic voltammogram of at 0.22 V, which is relatively more negative to that of
methanol oxidation. Fig. 5(c) shows the cyclic voltam- the other catalysts. This may be attributed to the high dis-
mogram of Pt/N-CNT electrode in 1 M H2SO4/1 M persion of platinum catalysts and the nitrogen functional
CH3OH run at a scan rate of 50 mV sÀ1. The electrocat- groups on its surface. The higher electrocatalytic activity
alytic activity of methanol oxidation at the Pt/N-CNT of the nitrogen containing carbon nanotube supported
electrodes was evaluated and compared with that of electrode is due to higher dispersion and a good interac-
the conventional electrodes. During the anodic scan, tion between the support and the Pt particles The Vulcan
Fig. 4. EDX pattern of Pt/N-CNT electrode.

6. 910 T. Maiyalagan et al. / Electrochemistry Communications 7 (2005) 905–912
Fig. 5. Cyclic voltammograms of (a) pure Pt; (b) Pt/Vulcan (E-TEK) and (c) Pt/N-CNT in 1 M H2SO4/1 M CH3OH run at 50 mV sÀ1.
Table 1 also been reported [38]. All these results indicate that
Electrochemical parameters for methanol oxidation on the various the nitrogen functionality on CNT influences the
electrodes
catalytic activity of the catalyst. The enhanced electrocat-
Electrocatalyst Methanol oxidation Forward peak current alytic effect of the nitrogen containing carbon nanotube
onset potential (V) density (mA cmÀ2)
vs. Ag/AgCl
supported electrodes could also be partly due to the
following factors which require further investigation:
Bulk Pt 0.4 0.167
20% Pt/C (E-TEK) 0.45 1.3
(1) higher dispersion on the nitrogen containing carbon
Pt/N-CNT 0.22 13.3 nanotube support increases the availability of an
enhanced electrochemically active surface area, (2)
appearance of the specific active sites at the metal–
support boundary and, (3) strong metal–support
carbon support has randomly distributed pores of vary- interaction.
ing sizes which may make fuel and product diffusion Long-term stability is important for practical applica-
difficult whereas the tubular three-dimensional morphol- tions. Fig. 6 shows the current density–time plots of var-
ogy of the nitrogen containing carbon nanotubes makes ious electrodes in 1 M H2SO4 and 1 M CH3OH at 0.6 V.
the fuel diffusion easier. The Vulcan carbon contains high The performance of Pt electrodes was found to be poor
levels of sulfur (ca. 5000 ppm or greater), which could compared to the E-TEK and Pt/N-CNT electrode. The
potentially poison the fuel-cell electrocatalysts [37]. nitrogen containing carbon nanotube electrodes are the
Nitrogen containing carbon nanotubes used in this study most stable for direct methanol oxidation. The increas-
contains heterocyclic nitrogen so that it preferentially at- ing order of stability of various electrodes is; Pt < Pt/
taches the Pt particles. The selective attachment of Au Vulcan (E-TEK) < Pt/N-CNT. We are currently investi-
nanoparticles on nitrogen doped carbon nanotubes has gating whether nitrogen has a catalytic role that contrib-

7. T. Maiyalagan et al. / Electrochemistry Communications 7 (2005) 905–912 911
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